The invention concerns a method for operating a wave power plant, in particular an oscillating-wave-column wave power plant, as well as a method for operating the same.
Wave power plants operating according to the oscillating wave column (OWC) principle are known. As an example reference is made to document U.S. Pat. No. 5,191,225. To do so, a wave chamber is used, wherein the water surface goes up and down according to the cycle of the waves. For that purpose, an access opening is arranged below the water surface for the entry of waves surging against the wave chamber. An air volume, situated in the wave chamber, is periodically put under pressure by the movement of the water surface, so that an oscillating air current is generated in a flow channel leading to the exterior. An air turbine is arranged in this flow channel, which typically rotates in a single direction for an incident air flow received from opposite directions, wherein in particular a Wells turbine comes into consideration.
Operating an electrical generator, using an air turbine of an OWC-wave power plant, rotating in a single direction, when receiving an incident air flow from opposite directions, raises the problem that due to the oscillation of the airflow in the flow channel originating from the wave chamber, the flow velocity significantly varies over an oscillation period. Additionally, stochastic fluctuations in the flow velocity do occur. This causes the turbine blades to stall, in particular for operating conditions with insufficient rotational speed of the air turbine and a high flow coefficient stalling occurs, which is accompanied by a strong power drop as well as high noise generation. Therefore, the aforementioned document U.S. Pat. No. 5,191,225 suggested to provide a runner of an air turbine with a large gyrating mass. Bringing such a runner to a high nominal rotation speed enables to operate the air turbine permanently with a low flow coefficient and hence reliably prevent it from stalling. Moreover, the high moment of inertia of the runner causes smoothing of the mechanical power transmitted to the electrical generator. The shortcoming of a runner with large gyrating mass is however that it cannot be driven under optimum power conditions over a wide operating range and that high ventilation losses may occur.
An alternative arrangement for operating an air turbine for an OWV-wave power plant consists in adapting the braking torque, applied by the electrical generator to the runner of the air turbine, in accordance with an averaged pressure in the wave chamber and moreover limiting braking torque to avoid stalling. Such an averaged adjustment of the braking torque, applied by the electrical generator, is possible in particular for electrical machines, operated with a frequency inverter. If the torque generated by the air turbine exceeds the braking torque of the electrical generator, the runner of the air turbine will be accelerated and consequently the flow coefficient is reduced. If the limitation of the braking torque of the electrical generator is adapted to the maximum flow velocities, occurring in the flow channel, the stalling behaviour can be improved. However, there is still the shortcoming that the air turbine does not reach its power optimum over wide operating ranges.
A frequency inverter with an integrated direct current link can be used for supplying electric power from an electrical generator, which is operated with variable rotational speed, to a power grid with constant frequency. A corresponding grid connection of variable speed wind power turbines with a synchronous generator is known. The energy storages used for this application enable a certain degree of smoothing of the power fed in a power grid, but they are inappropriate to prevent a pulsing power feed of a generic wave power plant. As regards the grid connection, reference is made to the scientific article CHILDS J. F “The role of converters and their control in the recovery of wave energy”, 19970616, 16 Jun. 1997 (1997-06-16), pages 3/1-3/7.